Protein kinases constitute a large and diverse family of enzymes. One member is the catalytic (C) subunit of cyclic adenosine monophosphate (cAMP) dependent protein kinase (cAPK). In the absence of cAMP, the enzyme is sequestered as an inactive tetrameric complex containing both regulatory (R) and catalytic subunits. Upon binding of cAMP, the holoenzyme (R2C2) dissociates into an R-cAMP dimer and two active, monomeric C subunits. Recently, we have solved structures of the rC-subunit complexed with adenosine and for the first time without peptide inhibitor. Also, we have solved the structure of the rC-subunit with a potent natural product inhibitor. These structures are beginning to lay the basis for structure based drug design. The structure solution of this apo form would greatly enhance our knowledge on the domain movements, protein conformational changes and also give better starting points for drug design.